Heretofore, as methods for the formation of a resistive metal oxide film for use in hybrid integrated circuit and various electronic apparatus there have been well-known a thick film-forming process which comprises screen-printing on a substrate a paste obtained by mixing a mixture of a metal and/or metal oxide powder and glass with a resin solution as a binder, and then calcining the material to form a film thereon and a thin film-forming process utilizing the sputtering of a resistive element film-forming material.
In the former process, as disclosed in JP-A-53-100496 and 54-119695 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a thick resistive element film-forming paste comprising a mixture of a ruthenium oxide powder and a glass frit powder dispersed in an organic vehicle made of a mixture of a solvent and a resin is screen-printed on a substrate, and then calcined to form a resistive element thereon.
In the latter process, as disclosed in JP-A-55-63804, vacuum technique is applied. A thin film of a sparingly soluble metal such as tantalum is deposited on a substrate by a sputtering process, and a pattern is then formed by a photolithographic process to form a thin resistive element film thereon. This resistive element can be used for some kinds of thermal heads.
The former thick film-forming process with the conventional thick resistive element film-forming paste requires an inexpensive apparatus for forming resistive elements and provides a high reproducibility. However, this thick film process has disadvantage in that the resulting resistive element film has a thickness of about 10 .mu.m or more. Furthermore, this process has disadvantage in that since the thick resistive element film-forming paste is an ununiform mixture of a glass frit powder and a ruthenium oxide powder, the resulting resistive value varies widely or the strength to electric field is low, that is, when the voltage applied is altered, the resistive value suddenly changes. Moreover, this process has disadvantage in that it is difficult to control the resistive value of the resulting resistive element by the composition ratio of a glass powder and a ruthenium oxide powder alone, and the difference in grain diameter between a glass powder and a ruthenium oxide powder or the variation of calcining temperature causes a great dispersion of resistive value. Even if the composition ratio and the average grain diameter are kept constant, the resistive value of the resulting resistive elements are different by lot.
The latter thin film-forming process can provide a uniform thin film resistive element. However, this process requires an expensive apparatus and provides a low reproducibility.
Heretofore, various techniques have been proposed for the preparation of thin resistive element films using the above mentioned thick film-forming process with an inexpensive production apparatus. One of these proposed techniques is MOD (Metallo Organic Deposition) process. MOD process is similar to the thick film-forming process. In MOD process, an organic metal compound solution is used instead of a mixture of metal and/or metal oxide and glass to prepare a paste from which a thin film is then formed (as disclosed in JP-A-60-102701, JP-A-60-102702, JP-A-62-292453, JP-A-1-152074, JP-A-2-39953, JP-A-2-33901, and JP-A-2-33902).
As another MOD process there has been known a process which comprises coating a solution containing an organic metal compound on a substrate, and heating and calcinating the material to cause the material to decompose to obtain a thin film of the corresponding metal oxide or the like (as disclosed in JP-A-64-54710, JP-A-1-286402, and JP-A-1-220402). It has been known that an iridium compound is used as an electrically-conductive component for thin resistive element film-forming material in this MOD process.
The above mentioned thick film-forming process using an organic metal compound solution has disadvantage in that the preparation of a paste suitable for screen printing finds difficulties in viscosity or storage stability. Thus, a proper viscosity adjuster is required to prepare a paste with an optimum viscosity and excellent storage stability. For example, as a viscosity adjuster builder for adjusting the viscosity of an electrically-conductive film-forming paste there has been known a cellulose compound such as ethyl cellulose (as disclosed in JP-A-56-5354, JP-A-57-27505, and JP-A-58-19813). Some resistive element films are prepared with asphalt as a viscosity adjuster. However, these resistive element films comprise a glass powder besides an organic metal compound solution to maintain proper film-forming properties (as disclosed in JP-A-50-30094).
Ethyl cellulose and the like to be used as a viscosity adjuster for screen printing paste in the formation of a thin film by the above mentioned thick film-forming process using an organic metal compound solution exhibit a poor compatibility and film-forming properties depending on the organic metal compound. The above mentioned resistive element film-forming paste with asphalt as a viscosity adjuster, which comprises glass powder besides an organic metal compound solution to maintain proper film-forming properties, provides a resistive element film with a poor uniformity resulting in a dispersion and in the resistive value of the resistive element film.
Furthermore, an iridium-containing resistive element film obtained according to MOD process which has heretofore been known exhibits only a relatively low resistive value. The resulting resistive element film cannot be used in integrated circuits for high voltage.
In order to accomplish the above mentioned objects, the inventors made an extensive study on what causes the dispersion in the resistive value of these resistive elements. As a result, the inventors suggested that the dispersion in the resistive value is mainly caused by two factors, that is, dispersion in the film thickness of the resistive element and ununiformity of properties related to the physical properties of thin film such as material composition of the resistive element.
It is considered that the dispersion in the film thickness of the resistive element is caused by the dispersion in the film thickness which has occured upon printing of the resistive paste and remained after calcination. Accordingly, it is necessary to solve problems causing the dispersion in the film thickness upon printing such as uneven printing of the resistive paste.